Generally, in generators for vehicles, the generated voltage level is usually set (for example, at 14.5 volts) higher than the full charging voltage of a battery for ensuring a sufficient voltage level for charging the battery and for suitably supplying the electric load. The generated output of a generator for a vehicle varies according to the state of use of the electric loads, and the output of an engine which drives the generator also changes in proportion the use of such loads. But, in the case where the electric load amount is small or the battery is in a fully charged state, it is possible to supply a light load current sufficiently even if the generated voltage of the generator is dropped to the full charging voltage (e.g., 12V) of the battery. By so doing, moreover, not only is it possible to prevent the battery from being overcharged, but also the generator is relieved of its burden of overcharging so that the driving or power loss of the engine can be diminished. Thus, the improvement of the power generation efficiency can be attained.
FIG. 1 shows an output current characteristic of a generator for a vehicle versus the engine speed, taking the generated voltage as a parameter. As the generated voltage increases, the amount of generated power increases and the battery charging rate becomes higher. By utilizing this characteristic, if the power generation is performed at a lower voltage level under a light electric load condition which is an ordinary or usual state, the charging current decreases and the engine load is reduced, thus preventing the battery from being overcharged. On the other hand, under a heavy electric load condition, the generated voltage level is changed to a higher level to increase the amount of generated power, whereby not only the load requirement can be fully satisfied, but also it is possible to prevent overdischarge of the battery.
More particularly, there has heretofore been adopted such a control means as shown in FIG. 2 in which an output current I.sub.G of a generator 4 is detected by a current detector 5, and when the detected value is smaller than a preset level, the excitation of the generator 4 is weakened by a voltage regulator 6 to bring the generated voltage down to the full charging voltage of a battery 2.
However, such a conventional method of controlling the generated voltage according to the output current I.sub.G of the generator 4 is disadvantageous in that the control cannot be done stably because the output current I.sub.G of the generator 4 varies largely, depending on the state of an electric load 1, the speed, i.e., the number of revolutions, of an engine 3, the charging state of the battery 2, and so forth. For example, as shown in FIG. 3, upon starting of the engine, a current of 3 amps or so is supplied in the ignition system, which current is fed by the generator. Thereafter, if a large continuous load such as headlight or air conditioner is added at time point "a", the amount of generated power of the generator increases to make up the deficiency and its output current I.sub.G exceeds a level L set at 6 amps or so, and the generator is switched to a high generation mode (e.g., 14.5 V) (the portion A in FIG. 3). During running, if the engine slows down and its speed approaches an idling state (e.g., 700 rpm) at time point "b", the amount of generated power of the generator decreases and the output current I.sub.G becomes lower than the preset level L, whereby the generator is switched to a lower generation mode (e.g., 12 V) (the portion B in FIG. 3). As the engine speed increases and approaches a cruising state (e.g., 1500 rpm) at time point "c", the generator is again switched to the high generation mode (the portion C in FIG. 3). Thus, even when the electric load condition is constant, the output current I.sub.G of the generator is changed by other conditions such as, for example, the engine speed and the charging state of the battery, and the switching frequency of the preset voltage of the generator is increased by those other conditions, so that the engine output varies at every switching of the preset voltage, thus possibly affecting the power output of the engine.
There has also been proposed a control means as shown in FIG. 4 in which a battery current I.sub.B is detected by a current detector 5, and the generated voltage level of a generator 4 is switched to a high or low level while the detected value is compared with a preset level by a voltage regulator 6. Also in this case there is the problem that even when the applied load is constant, the switching frequency of the preset voltage of the generator 4 is increased by other conditions. For example, as shown in FIG. 5, even if a continuous load is applied at time point "a" when the engine speed is high, there will be no change of a battery charging current I.sub.BC, but when the engine slows down at time point "b", the battery starts discharging because with only the output current of the generator it becomes no longer possible to supply all the load current. When a discharging current I.sub.BD of the battery exceeds a preset level L', the generator is switched to a high generation side (the portion B in FIG. 5). Thereafter, when the engine speed again increases at time point "c", the generator is switched to a low generation side because the output current of the generator increases sufficiently to supply all the load current. The mark "t" in FIG. 5 denotes an amount corresponding to the discharged amount.
It is accordingly an object of the present invention to provide an improved form of control system for an engine-driven generator.
Another object of this invention is to provide a control system for an engine-driven generator having improved power generation efficiency.
It is another object of this invention to provide a control system for an engine-driven generator which is capable of switching the generated voltage level of the generator in a stable manner according only to the state of the load applied.
These and other objects and features of the present invention will become better understood through a consideration of the following description taken in conjunction with the various figures of the drawings.